1. Field of the Invention
This invention relates generally to image processing machines and more particularly to apparatus and methods for translating a source image having a first resolution to a display image having a second, relatively lower resolution. The invention is particularly applicable to a projection display apparatus utilizing a liquid crystal display panel.
2. Description of the Related Art
Images in the form of pictures, graphics, background and text can be displayed on a variety of equipment including cathode ray tube (CRT) systems used in television receivers and computer monitors, light emitting diode (LED) displays, plasma display panels, liquid crystal displays (LCDs), etc. or can be printed using a variety of equipment including ink-jet and laser printers. Each of these image output devices will display or print the image at a spatial resolution defined by the physical parameters and limitations of the equipment. On a CRT display, for example, light is produced by three primary phosphors, red, green and blue (RGB), which are excited separately by the electron beam in the CRT. The resolution of the CRT will vary with the higher cost display screens providing the greatest resolution. For example, a computer VGA (black and white) screen may have a resolution of 640.times.480 (pixels/line.times.lines/image) with 8 bits/pixel precision while a computer SVGA (color) may have a resolution of 1024.times.768 with 16 bits/pixel precision.
In a projection display system, for example, white light from a lamp, such as halogen, metal halide or xenon lamp, is separated by optics, such as dichroic mirrors, into red green and blue (RGB) components. The RGB light beams are modulated by respective liquid crystal display (LCD) panels that are controlled by drive circuits according to video information. The modulated RGB beams are recombined by a prism unit, for example, and enlarged by a projection lens unit for projection onto a viewing screen. Like CRTs, the resolution of the LCD panels vary according to cost and certain of the higher end LCD large screen panels will have a resolution of about 1024.times.768 (number of dots, horizontal.times.vertical) while smaller panels will have a resolution of 312.times.230.
The source image may also come from a variety of input devices such a scanner, digital camera or computer. When the source image has a greater spatial resolution than the output device, down-sampling filters are often used to reduce the spatial resolution of all or some of the source image. Depending on the output device and the nature of the source image, the information lost in the reduced resolution output may or may not be noticed by the viewer.
Consider a specific example. The source image that is input to an LCD projector can be from a personal computer that is connected to an Input/Output (I/O) jack of the projector or may be stored on a floppy disc or hard diskette that is received and read by a diskette drive in the projector. In either case, the source image supplied by the computer or stored on disk is often in a format for a computer monitor that may have a resolution that is greater than that of the LCD projector. In this case the input video is low-pass filtered (down-sampled) to match the resolution of the projector. In certain instances the entire source image is treated the same, i.e. down-sampled using the same filter and filter coefficients. This can be effective in minimizing aliasing effects since high spatial frequencies are removed. While this may be acceptable for a source image that contains only lower frequency graphics and background information, it presents a problem for an image having a mixture of graphics and text. In such instance, some of the high frequency content is lost in the output image, and the text is degraded with portions of characters missing. When this image is then projection enlarged onto a screen, such degradation is noticeable and obvious to a viewer.